Why Looking Inside Tank Is Worst Way To Evaluate Your Industrial Mixer

Author: Sara Peters | October 15, 2015 | Category: Mixing

mixing_without_baffles“I look inside the tank, that’s how I know it’s mixed.” That’s how many operators determine when a batch is ready. But in actuality, it’s the worst way to know. Visual observation only gives you part of the story. How can you find out if you’re getting the best mix?

WHEN I LOOK IN THE TANK, I CAN JUST TELL ITS MIXED

What does looking at the top of the tank tell you? Sure, you may see a lot of surface motion and great things happening at the top, but what’s happening on the bottom? Are solids settling out? Is the bottom of the tank getting as much agitation as the top? Could you be getting an ever better mix, resulting in not only better product, but perhaps even less time spent mixing?

non-miscible-fluidsWHAT DOES MIXING MEAN?

Before we go any further, it’s important to understand what it means to have product completely mixed. Mixing can be defined “as the reduction of inhomogeneity in order to achieve a desired process result.” (Paul, Obeng, Kresta – NAMF) That “desired process result” could mean any of the following:

  • Blending of miscible liquids
  • Solids suspension
  • Dispersion
  • Dissolving
  • Crystallization
  • Heat transfer
  • Chemical reaction
  • Extraction
  • Gas dispersion

Mixing is achieved by the imposition of torque from the primary mover, resulting in two effects:

  • Thrust – which is reflected as flow
  • Shear – Transfers mechanical energy into heat

blendingHOW DO I KNOW I’M GETTING THE BEST MIX?

So, understanding how mixing is achieved, how do you know if your mix is the most efficient?

First, ask yourself, is the fluid you’re mixing today what the industrial mixer and tank were originally designed for? If not, there’s a very good chance the answer to the question is no, you’re not getting the best mix.

Next, since a big part of getting the thrust and shear you require is getting the right amount of torque, check the motor on your industrial mixer. How much power is being transferred? Is it using the amount noted on its tag?

What about batch time? Is the mixing time living up to what was originally designed?

Lastly, if you really like math, you can calculate impeller tip speed to estimate the shear applied to the fluid. Use the calculation below. Tip speed is expressed as velocity in feet/second.

V(ft/sec) = RPM x D(in) x π/720 

If you find out that your industrial mixer is no longer meeting the standards originally set up in design, it’s time to make some changes. Get an experienced engineer involved who can get your mixer back on track, help deliver more consistent product, and save you time and money in the process.

Think your industrial mixer needs to be re-evaluated? Ask us about it! We gladly provide technical assistance to businesses and municipalities in Wisconsin and upper Michigan.

Plant Engineer's Guide to Mixing and Agitation

Sara Peters

Sara Peters

Sara leads Crane Engineering's blogging team, coming up with fresh stories and insights for our readers to apply to their every day work.

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